About four years ago, the President of Harvard University, Larry Summers, was pilloried for daring to suggest that gender differences might play a role in determining the still-remaining gaps with regards to productivity, pay and proportion of representation between men and women in certain occupations. From a strictly scientific perspective, there was nothing inappropriate about his comments. However, it is virtually impossible to have a serious discussion of gender differences without running afoul of the preconceptions of the politically correct. However, it is important to do so, not least because genuine understanding of the differences between men and women is at least as important to women as it is for men. This is particularly true for young, competent women, who are trying to determine how to simultaneously maintain an all-consuming career, an intimate relationship, and a family.

I therefore want to draw my readers' attention to a paper published by David Schmitt and his colleagues in the Journal of Personality and Social Psychology in 2008 - a paper which should have been front page news, in my estimation, for what it revealed. The paper is entitled, provocatively: "Why Can't a Woman be More Like a Man? Sex Differences in Big Five Personality Traits Across 55 Cultures." It should be noted that this paper was published in an exceedingly prestigious journal, and was not drifting along on some academic backwater.

Academics in the 1960's were more or less universally convinced that personality and cognitive differences between the genders were environmentally produced, and that they would, in consequence, disappear, as the social environment became more homogeneous and less discriminatory. This theory was flawed, conceptually, for two reasons: First, there was no reason to assume, a priori, that all the differences were environmental, to begin with. Second, when environmental variance disappears, because of increasingly "fair" treatment, the only factors left that determines differences between people are genetic. The theory also appears to be flawed for reasons of fact.

Schmitt's paper addresses this issue head-on. He analyzed 18000 responses to a well-designed personality questionnaire measuring extraversion (social dominance, positive emotion, gregariousness), negative emotionality (sensitivity to punishment and threat, manifested as pain, fear, guilt, shame, anger and self-consciousness), agreeableness/assertiveness, conscientiousness (integrity, orderliness and persistence) and openness (creativity and intelligence). The questionnaire was administered to participants in 55 nations. The results were crystal clear, and they were not favorable to those holding the simple-minded environmental hypothesis. The more developed the nation and, by any reasonable standard, the more egalitarian, the larger the personality differences manifested between men and women.

What this means, essentially, is that men and women will differ for innate, genetically predicated reasons, when their environments approach equality of opportunity. Furthermore, these differences are large and important, and stable across the developed countries where they were most likely to be manifest.

Before I tell you what those differences were, I want to point out some obvious facts about men and women. First, they differ in size. The typical woman weighs about 140 pounds and is 65 inches in height at age 30, while the typical man weighs about 180 and is 70 inches in height. Thus the typical man weighs about 30% more and is about 8% taller. Men are also much stronger in their upper bodies than women, and have broader and more heavily developed jaws.

Among animals characterized by such sexual dimorphism (difference between the genders), the larger gender tends to be more aggressive. Sometimes, as in the famous case of spotted hyenas, the female is larger and more aggressive. She pays for this, by the way, by suffering the requirement to give birth (rather painfully, by all evidence) through an organ that is virtually indistinguishable in size and shape from the male hyena penis.

Schmitt's paper demonstrates quite clearly that the personality differences that would be expected between the genders given their sexual dimorphism exist, are large in size, and are more prevalent in developed countries: males are much more assertive (less agreeable) and much less prone to negative emotion.

This is precisely in keeping with a very large body of clinical research demonstrating that women are more likely, cross-culturally, to suffer from anxiety disorders and depression (4:1), as would be expected given their higher sensitivity to negative emotion, and that men are far more likely (16:1 in the US) to be incarcerated.

Why might these differences exist? Perhaps women are more prone to negative emotion than men because life is and has always been simply more dangerous for women. They are smaller, physically weaker in the particular ways that are associated with fighting (where upper body strength plays a large role), and prone to sexual assault, which has tremendously devastating costs, psychologically, physically, and practically. It is interesting in this regard to note that the differences in sensitivity to negative emotion also seem to emerge at puberty (as boys and girls are more similar than men and women), when girls become sexually mature and the target of often predatory attention. It may also make sense for women to be more sensitive to threat than men, because they generally are charged with the care of very young children, who are vulnerable and easily damaged, and who need to be protected from dangers that pose no problem to adults.

Men and women are different. These differences remain, even grow, in egalitarian cultures. The differences are large and important. In occupations that are very stressful and competitive, women are therefore typically at a pronounced psychological disadvantage, regardless of their equivalent intelligence and conscientiousness (the two best predictors of competence), as they will suffer more per unit of uncertainty, and because it will be more difficult for them to withstand the competitive pressure put on them by the extremely dominant, aggressive and anxiety-resistant men who typically occupy such positions.

It is a dirty little secret in the corporate and professional worlds that women in their thirties tend disproportionately to disappear from extremely competitive, high-stress occupations (the practice of law in large law firms is particularly affected). This has nothing to do with their competence, or with the firms, in my opinion, as law firms are desperate for highly competent people, regardless of gender. It is simply that highly educated women in their thirties are often looking to start a family, have partners who make plenty of money, and start wondering "why in the world am I working 80 hours a week?" This is an eminently reasonable question, and there is no reason in the world to assume that any job, no matter how high-powered, is necessarily worth the sacrifice of life with small children.

The real question, therefore, is not "why do many women choose not to work eighty hours a week during their thirties" but "why are a small but very persistent minority of men willing to do so?" I will post some thoughts about that later.